Acetolactate synthase (hereinafter referred to as “ALS”) is a rate-determining enzyme in the biosynthetic pathway of branched chain amino acids, such as leucine, valine and isoleucine, and is known as an essential enzyme for the growth of plants. ALS is also known to be present in a wide variety of higher plants. In addition, ALS is found in various microorganisms, such as yeast (Saccharomyces cerevisiae), Escherichia coli, and Salmonella typhimurium. 
Three types of isoenzymes of ALS are known to be present in Escherichia coli and Salmonella typhimurium. Each of these isoenzymes is a hetero oligomer consisting of catalytic subunits with a large molecular weight that governs catalytic activity of the enzyme and regulatory subunits with a small molecular weight that functions as regulatory proteins by binding of branched-chain amino acids (Chipman et al., Biochim. Biophys. Acta. 1385, 401–419, 1998). Catalytic subunits are located at Ilv IH, Ilv GM and Ilv BN operons, respectively. On the other hand, ALS in yeast is a single enzyme, which comprises a catalytic subunit and a regulatory subunit, as is the case in bacteria (Pang et al., Biochemistry, 38, 5222–5231, 1999). The catalytic protein subunit is located at the locus ILV2.
In plants, ALS is known to comprise catalytic subunit(s) and regulatory subunit(s) as is the case in the above microorganisms (Hershey et al., Plant Molecular Biology. 40, 795–806, 1999). For example, the catalytic subunit of ALS in tobacco (dicotyledon) is encoded by two gene loci, SuRA and SuRB (Lee et al., EMBO J. 7, 1241–1248, 1988); that in maize is encoded by two gene loci, als 1 and als 2 (Burr et al., Trendsin Genetics 7, 55–61, 1991; Lawrence et al., Plant Mol. Biol. 18, 1185–1187, 1992). The nucleotide sequences of genes encoding a catalytic subunit have been completely determined for dicotyledonous plants including tobacco, Arabidopsis, rapeseed, cotton, Xanthium, Amaranthus and Kochia (See Chipman et al., Biochim. Biophys. Acta. 1385, 401–419, 1998 and domestic re-publication of PCT international publication for patent applications WO97/08327). However, maize is the only monocotyledonous plant whose nucleotide sequence has been completely determined.
Herbicides, for example, sulfonylurea herbicides, imidazolinon herbicides, triazolopyrimidine herbicides and pyrimidinyl carboxy herbicides (hereinafter referred to as “PC herbicides”) are known to suppress the growth of a plant by inhibiting ALS (Ray, Plant Physiol. 75, 827–831, 1984; Shaner et al., Plant Physiol.76, 545–546, 1984; Subramanian et al., Plant Physiol. 96, 310–313, 1991; Shimizu et al., J. Pestic. Sci.19, 59–67, 1994).
Known plants having resistance to these herbicides contain a gene encoding ALS that includes substitution of one or two nucleotides which induces substitution of one or two amino acids in a region conserved among different species. Examples of such a gene include a gene encoding ALS having resistance specific to sulfonylurea herbicides (see Kathleen et al., EMBO J. 7,1241–1248, 1988; Mourad et al., Planta, 188, 491–497, 1992; Guttieri et al., Weed Sci. 43,175–178, 1995; Bernasconi et al., J. Biol. Chem. 270, 17381–17385, 1995 and Japanese Patent Application Laying-Open (kokai) No. 63-71184); a gene encoding ALS having resistance specific to imidazolinon herbicides (Mourad et al., Planta, 188, 491–497, 1992; Lee et al., FEBS Lett. 452,341–345, 1999 Japanese Patent Application Laying-Open (kokai) No. 5-227964); and a gene encoding ALS having resistance to both sulfonylurea and imidazolinon herbicides (see Kathleen et al., EMBO J. 7,1241–1248, 1988; Bernasconi et al., J. Biol. Chem. 270, 17381–17385, 1995; Hattori et al., Mol. Gen. Genet. 246,419–425, 1995; Alison et al., Plant Physiol. 111, 1353, 1996; Rajasekarau et al., Plant Sci. 119, 115–124, 1996, Japanese Patent Application Laying-Open (kokai) No.63-71184, Japanese Patent Application Laying-Open (kokai) No.4-311392 and Bernasconi et al., U.S. Pat. No. 5,633,437, 1997). ALS showing resistance to both sulfonylurea and imidazolinon herbicides is also known to show cross resistance to PC herbicides and triazolopyrimidine herbicides (Bernasconi et al., J. Biol. Chem. 270, 17381–17385, 1995). Further, the production of a plant body showing resistant to both sulfonylurea and imidazolinon herbicides has been attempted by crossing a plant having ALS showing resistance specific to sulfonylurea herbicides with a plant having ALS showing resistance specific to imidazolinon herbicides (Mourad et al., Mol. Gen. Genet, 243, 178–184, 1994). Furthermore, artificial alteration of a gene encoding ALS into a herbicide resistance gene has been attempted (Ott et al., J. Mol. Biol. 263, 359–368, 1996, Japanese Patent Application Laying-Open (kokai) No. 63-71184, Japanese Patent Application Laying-Open (kokai) No. 5-227964, Japanese Patent Application Laying-Open (kohyo) No. 11-504213) so that it has been found that a single amino acid deletion causes ALS to show resistance to both sulfonylurea and imidazolinon herbicides (see Japanese Patent Application Laying-Open (kokai) No. 5-227964).
As described above, ALSs having resistance to herbicides and genes encoding ALS have been aggressively studied. However, there has been no report directed toward resistance to PC herbicides and concerning a mutant ALS gene having resistance specific to a PC herbicide.